As disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-298940, it is possible to prepare silicone resins having any arbitrary molecular weight, softening point, or glass transition points depending on the reaction conditions and the combination of siloxanes or silanes used as the starting materials, however, regulating their melt viscosity and their reactivity and dispersibility in mixtures with organic resins has been a tough problem.
On the other hand, although curable resin compositions can produce cured resin of superior dielectric characteristics, volume resistivity, dielectric breakdown strength, and other electrical characteristics, as well as flexural strength, compression strength, impact strength, and other mechanical characteristics, in order to improve their flame retardant properties, they have to be combined with halogen-containing compounds and antimony oxides, such as antimony trioxide, which gives cause for concern with respect to their impact on the human body and the environment due to the toxicity of the antimony oxide powders and the toxic gases released during the burning of the resultant cured resins.
In accordance with the teachings of Japanese Unexamined Patent Application Publication No. Hei 6-298897, combining silicone resins with curable resins improves pre-curing flowability and provides for better flexibility, moisture resistance, and resistance to thermal shock. In addition, in accordance with the teachings of Japanese Unexamined Patent Application Publication No. Hei 11-222559 and Japanese Unexamined Patent Application Publication No. Hei 11-323086, etc., combining silicone resins with curable resins improves the flame retardant properties of the cured resin. However, the problem with the curable resin composition disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-298897 was the insufficient flame retardant properties of the resultant cured resin. The problem with the curable resin compositions disclosed in Japanese Unexamined Patent Application Publication No. Hei 11-222559 and Japanese Unexamined Patent Application Publication No. Hei 11-323086 was poor moldability causing mold contamination during molding, etc.
It is an object of the present invention to provide a silicone resin composition of low melt viscosity and excellent reactivity and dispersibility in organic resins, a curable resin composition forming a cured resin of excellent moldability and superior flame retardant properties and having little adverse impact on the human body and the environment as a result of containing no antimony oxides or halogenated epoxy resins, as well as a cured resin having little adverse impact on the human body or the environment and possessing superior flame retardant properties.